regular polygons

Help me /prog/, i'm stuck on exercise 2.2 of the haskell school of expressions.

Define a function regularPolygon :: Int -> Side -> Shape such that regularPolygon n s is a regular polygon with n sides, each of length s.

I've found a couple answers, but don't get them.

regularPolygon :: Int -> Side -> Shape
regularPolygon n s
    = let angleinc = pi * 2 / fromIntegral n
          radius = s * sin ((pi - angleinc) / 2) / sin angleinc
          regularVerts 0 _ = []
          regularVerts n angle = (radius * cos angle, radius * sin angle)
                                 : regularVerts (n-1) (angle + angleinc)
      in Polygon (regularVerts n 0)

are you aware there are many many ways to get a regular polygon with n sides?
anyway just think of a circle centered at 0, divide 2*pi with the number of sides, so now you got the vertexs, but you have to muliply them for some factor to get the desired side length.
but you've got a triangle, you know the sides and one angle, so you can calculate the other side length.
But it seems like you've already figured out this, so what is getting you stuck?